Method for the preparation of hollow polymer particle latex

ABSTRACT

A method for the preparation of latexes with hollow polymer particles useful as opacifying agents. The process comprises the preparation of hollow polymer particle latex by emulsion copolymerization with water soluble initiator and anionic surfactant as follows: a) preparation of seed latex of copolymer containing methyl methacrylate and methacrylic acid; b) preparation of latex of highly carboxylated copolymer containing methyl methacrylate and a cross-linking agent and optionally a vinyl aromatic compound; c) using highly carboxylated latex synthesized at stage b) in the preparation of the intermediate shell comprising a copolymer of methyl methacrylate, acrylic acid ester, a cross-linking agent and optionally vinyl aromatic compound, said intermediate shell copolymer having a glass transition temperature below 80° C.; d) swelling the particles prepared at stage c) by addition of volatile basic compound; e) preparation of a hard shell on the swollen expanded particles comprising a copolymer of vinyl aromatic compound, acrylonitrile and/or methyl methacrylate, and cross-linking agent, said hard shell having a glass transition temperature above 80° C.; and f) optional preparation of an external shell.

This application is the national phase under 35 U.S.C. §371 of PCTInternational Application No. PCT/FI98/00194 which has an Internationalfiling date of Mar. 4, 1998, which designated the united states ofAmerica.

The present invention relates to a method for the preparation of latexeswith hollow polymer particles. Hollow polymer particles are useful asopacifying agents in coating applications, such as in paint or papercoating, especially for high quality paper products where colourpictures are presented, such as art books, brochures, annual reports,magazines etc. The use of hollow particle latex in such coatings reducesor removes the need for pigments, such as titanium dioxide, orextenders, without adding excessive and undesirable weight to thecoating. The hollow polymer particle latex provides opacity because thehollow structure in a latex particle scatters light more efficientlythan a corresponding particle of uniform density. The light scatteringproperties are related to a difference in the refractive index betweenthe shell and the internal void. The particles can also be used ascontrol release devices for target compounds. Hollow polymer particleshave found extensive applications as white pigments, ie. hollow spherepigments have excellent optical properties such as hiding power,opacity, gloss, brightness and whiteness as well as thermal resistanceproviding benefits in printing performance and production efficiency,for example faster calendering. Further, they have started to replacethe earlier used polymers filled uniformly in particles.

In the prior art, some methods have been known for the production ofmicrovoid containing polymer particles. All these methods lead to a coreand shell particles with different microvoids in the center. The hollowpolymer latex particles having a centered void have a greater utility incoatings than those structures having microvoid or hemisphericalparticles with voided centers.

Some methods have also been proposed for the preparation of hollowpolymer particles. EP patent number 22633 discloses a process whichcomprises copolymerizing an ethylenically unsaturated monomer with acarboxyl group containing ethylenically unsaturated monomer by emulsionpolymerization to prepare seed particles of the core part, carrying outcover polymerization using an ethylenically unsaturated monomer for theformation of the particle surface layer eg. shell part, neutralizingafter completion of polymerization the resulting particles with avolatile base such as ammonia, and swelling the particles into hollowparticles. In this process, four-stage polymerization is carried out soas to make a polymer composition of the core part and the shell partdifferent. The process is complicated and it is difficult to control thereaction temperature, the quantity of the polymerization initiator, thequantity of the surfactant etc. Thus, it is difficult to form hollowparticles stably, resulting in high levels of copolymerized acidremaining in the core polymer. There also appears to be no intermediarylayer between the core and the shell, which may be the reason for theweak opacity.

U.S. Pat. No. 4,427,836 discloses the production of sequentialheteropolymer dispersion and a particulate material obtainabletherefrom, comprising an ionic, swellable, hard core polymer and asheath polymer on the core. In this method, the preparation of thecore/sheath polymer particles is realized using sequential emulsionpolymerization to yield the core, then polymerizing the sheath on thecore particles, neutralizing the resultant core/sheath particles toswell the core and to form particles with a single void when dried. Theproduct contains only two layers, the core and the sheath. This may bethe reason why the opacifying compositions are only whitish, but notwhite.

U.S. Pat. No. 4,880,842 discloses a process for making hollow latexes byintroducing a non-polymeric acid to an early stage of the multi-stagepolymer particles instead of copolymerizing acid to make swellablecores. This method for preparing hollow latexes requires corescontaining acid or acidic monomers to enable swelling to occur at roomtemperature. The existence of acid in a polymer disadvantageouslyincreases the polymers' affinity to water. The sensitivity to theeffects of water undesirably renders the coating into which the latex isformulated, sensitive to moisture as well, which is an undesirableproperty in coating applications. An acidic core is also thought toimpart polarity to the polymer and because of such polarity the polymeris thought not to absorb surfactants well, rendering the polymerunstable or prone to coagulation or aggregation in larger particles. Thewater sensitivity of the core or high affinity for water by the core isthought to make it difficult to encapsulate the core with the shellpolymer because the core tends to remain at the polymer-water interfaceon the surface of the particle rather than to be encapsulated by theshell. This process for preparing hollow polymer particles also requiresa post-encapsulating step with cross-linked polymers, that is the acidcontaining cores are encapsulated with an uncross-linked shell, the coreis swelled, and then the shell can be encapsulated with cross-linkedpolymers. Additionally, substantive amounts of the chain-transfer agentn-dodecyl mercaptan are needed to accomplish this process.

U.S. Pat. No. 4,910,229 discloses a process which comprises adding abase to a carboxyl group containing copolymer latex to neutralize atleast part of the carboxyl groups in the copolymer, eg. base treatmentand then lower the pH of the latex with an acid, eg. acid treatment toproduce hollow particles. In this process it is necessary to neutralizethe carboxyl groups in the polymer particles, and in order to make thebase necessary for neutralization permeate sufficiently into the polymerparticles, there is a necessity to raise treatment temperature from atemperature somewhat lower than the glass transition temperature (T_(g))of the polymer to a temperature equal to or more than T_(g) and raisethe diffusion rate of the base. Usually this kind of process results inmultihollow morphology of the particles.

U.S. Pat. No. 5,360,827 discloses a process for the preparation of latexof a hollow polymer which comprises adding a base in the presence of amonomer to latex containing carboxylic modified copolymer particles tomake the pH of the latex 8 or more, and adding a carboxyl groupcontaining a monomer to make pH of the latex 7 or less and thenpolymerizing these monomers. This process leaves high amounts ofresidual monomers in the product and thus, an additional steamdistillation operation is required.

U.S. Pat. No. 5,157,084 discloses a process of making hollow polymerlatex particles without the use of acid in the swellable polymer core.The swelling step is performed at elevated temperatures, usually above100° C. and thus, it must be performed under pressure.

The European patent application, publication No. 478 829 discloses aprocess for the preparation of hollow latex polymers characterized byemulsion polymerization of monomers in the presence of organichydrocarbons, which tend to leave solvent residues in the product andare costly to use. An additional operation is needed for the removal ofresidual solvents from the product.

In general, all methods in the state of the art are multistage processesrequiring the isolation of the intermediate latexes, several reactorsand time consuming expensive cleaning operations between the stages.

An object of the present invention lies in providing a new, efficient,economic and environmentally advantageous process for the preparation ofstable hollow polymer latex particles, based on the development of ahard, cross-linked shell on highly carboxylated seed particlespre-swelled in aqueous volatile base solution. Surprisingly the presentinventors found that the above-identified objects can be attained byrealizing the following process, which requires no chain transferagents, no additional steam distillation steps, no pressurizing, noorganic solvents, no elevated temperatures above 100° C., and whichyields a white and opaque product with an additional intermediary layer.The process can be performed batchwise in one reactor without anyisolation of intermediate latexes and without additional cleaning etc.operations.

Accordingly, the present invention provides for an efficient andeconomic process for the preparation of hollow latex polymer particleswith a high glass transition temperature T_(g), with particle sizebetween 0.3-0.6 μm and with suitable rheology.

The process comprises preparing hollow polymer particle latex byemulsion copolymerization with water soluble initiator and anionicsurfactant, using batch, semi-continuous batch or continuous batchmethods as follows:

a) preparation of seed latex of copolymer containing methyl methacrylateand methacrylic acid;

b) preparation of latex of highly carboxylated copolymer containingmethyl methacrylate and a cross-linking agent and optionally a vinylaromatic compound;

c) using highly carboxylated latex synthesized at stage (b) in thepreparation of the intermediate shell comprising a copolymer of methylmethacrylate, acrylic acid ester, a cross-linking agent and optionally avinyl aromatic compound, said intermediate shell copolymer having aglass transition temperature below 80° C.;

d) swelling the particles prepared at stage (c) by the addition of avolatile basic compound;

e) preparation of a hard shell on the swollen expanded particlescomprising a copolymer of vinyl aromatic compound, acrylonitrile and/ormethyl methacrylate, and cross-linking agent, said hard shell having aglass transition temperature above 80° C.; and

f) optional preparation of an external shell consisting of a copolymerof a vinyl aromatic compound, an acrylic acid ester, a methacrylic acidand a cross-linking agent with glass transition temperature below 15° C.

Stage (a) preferably comprises the preparation of a seed latex copolymercontaining methyl methacrylate and methacrylic acid and the optionalpreparation of the seed latex when weight ratio ofmonomer/initiator/water is equal to 1/(0.1-0.3)/(50-150).

The ratios of the weight concentrations of the copolymers preparedduring different stages are important parameters of the process.

Preferred ratios of the polymers obtained in different stages arepresented below.

Stage No a b c e f Preferred 1 5-20 25-60 20-60 2-60 ratio

The overall process is preferably performed in a single reactor as aone-pot process but it also may be performed as a conventional stepwiseprocess with the isolation of intermediate latexes using severalreactors.

At stage (a) seed latex is prepared by emulsion copolymerization of98.8-99.5 weight parts, preferably 97.0-98.5 weight parts of methylmethacrylate with 0.5-4.0 weight parts, preferably 1.5-3.0 weight partsof methacrylic acid.

When preparing the hollow latex polymer particle via a seeded process, aseed latex prepared at stage (a) having a particle size from 70 to 180nm is required, and the particle size range of 80-130 nm is preferred.

At stage (b), latex of highly carboxylated copolymer containing methylmethylacrylate, cross-linking agent and acid monomer is prepared byusing the seed latex prepared at stage (a) for obtaining copolymerlatex.

The preferred acid monomers that may be used for the alkali-swellablecore are monoethylenically unsaturated carboxylic acid monomers, suchas, acrylic acid, methacrylic acid and ithaconic acid. As the principalrole of the acid monomers at stages (a), (b), and (c), the acid monomersprovide the preparation of the carboxylated core, which is able to swellwhen the carboxylic groups are neutralized with a volatile base, atstage (f) the acid monomers primarily provide the stability of thelatex.

The cross-linking agent is meant to include monomers conventionallyknown in the art as useful for cross-linking polymerizable monomers.Examples of such monomers typically include di- or trifunctional monomersuch as divinyl benzene, ethylene glycol methacrylate, ethylene glycolacrylate and allyl methacrylate.

In the copolymerization of core polymer latex 60-80 parts, preferably65-75 parts of methyl methylacrylate and 20-40 parts, preferably 25-35parts of acid monomer, and 0.3-0.8 parts, preferably 0.4-0.6 parts ofcross-linking agentare used.

The acid monomers provide the preparation of carboxylated core which isable to swell when the carboxylic groups are neutralized at later stage(d) with volatile base. In addition, the acid monomers improve thecolloidal stability of the latexes, especially at stage (f).

At stage (c) the highly carboxylated latex prepared at stage (b) is usedfor the preparation of an intermediate shell on latex particles. Theintermediate shell comprises a copolymer of methyl methylacrylate,acrylic acid ester and cross-linking agent and said intermediate shellhas a glass transition temperature below 80° C. The acrylic acid estermay be, for example, n-butylacrylate, isobutyl acrylate or 2-ethylhexylacrylate.

The amount of monomers used at stage (c) are as follows: 60-85 weightparts, preferably 70-80 weight parts of methyl methacrylate, 15-25weight parts, preferably 16-19 weight parts of acrylic acid esters and2-8 weight parts, preferably 4-5 weight parts of acid monomers.Optionally, the monomer mixture contains also 0-0.6, preferably 0.1-0.3weight parts of cross-linking agents mentioned above.

At stage (d) the particles prepared at stage (c) are swelled by adding avolatile basic compound.

Volatile bases, such as ammonia, or volatile organic bases, such astriethylamine, trimethylamine or diethyl ethanol amine are used assuitable swelling agents.

The concentration of volatile basic compound at stage (d) is preferablycalculated in accordance with the following equation:

 [B]=a/D−b

where [B] is the volatile base concentration calculated asmilliequivalents per 1 g of methacrylic acid charged at stages (a), (b)and (c), D is number average diameter of the seed particles (innanometers) prepared at stage (a) and a and b are constants, ie. a=1670and b=4.1.

The volatile base concentration at stage (d) is preferably 2-10% byweight and more preferably 3.5-6% by weight.

The swelling time can be 30-120 min, preferably 40-80 min.

At stage (e), a hard shell having a glass transition temperature above80° C. is prepared on the swollen expanded particles from stage (c).This shell comprises a copolymer of vinyl aromatic compound, such likestyrene, acrylonitrile and/or methyl methacrylate and a cross-linkingagent. The monomers used at this stage are as follows: 0-35 weightparts, preferably 15-30 weight parts of methyl methacrylate, 60-99weight parts, preferably 70-80 weight parts of styrene, 0-35 weightparts, preferably 15-30 weight parts of acrylonitrile and 1-10 weightparts, preferably 2-5 weight parts of cross-linking agent mentionedabove.

An optional stage (f) comprises preparation of an extended shellconsisting of a copolymer of vinyl aromatic compound, an acrylic acidester, methacrylic acid and a cross-linking agent and having a glasstransition temperature below 15° C. This can be obtained bycopolymerizing 3-10 weight parts, preferably 4-7 weight parts ofmethacrylic acid, 55-75 weight parts, preferably 60-75 weight parts ofacrylic acid ester, 20-40 weight parts, preferably 30-35 weight parts ofstyrene and 0.2-3.5 weight parts, preferably 0.4-1.0 weight parts ofcross-linking agent.

Vinyl aromatic compounds, such as styrene, p-methylstyrene and mixturesof isomers of methylstyrene can optionally be used at stages (b) and(c). The amount of vinyl aromatic compound is 0-20 weight parts,preferably 0-10 weight parts. Further, the ratio of weightconcentrations of the sum of methyl methylacrylate and vinyl aromaticcompound to acrylic acid ester at stage (c) is preferably within therange of 3.3-4.3 and the ratio of weight concentration of the copolymerprepared at stage (f) to the copolymer prepared at stage (e) ispreferably 0.1-1.0.

The overall reaction temperature at all stages except stage (d) is60-95° C., preferably 80-90° C. The reaction temperature of stage (d) is80-95° C.

The water soluble free radical initiators suitably employable in theaqueous phase are those typically known in the art, for example sodiumpersulphate, potassium persulphate and ammonium persulphate. Theeffective amount of the initiator can be from 0.25 to about 3.00%,preferably 0.6-1.45% by weight of the monomer charged at all stages. Theinitiator is added at stage (b) advantageously as two equal portions inthe beginning of the reaction and in the middle of the reaction.

In all steps, surface active agents typically known in the art forpolymerization processes can be used. Typical surfactants includeanionic emulsifiers, such as, sodium dodecyl benzene sulphonates, sodiumalkyl sulphonates (C₁₂-C₁₈) in effective amounts to assist instabilizing the particle as a colloid minimizing contact between theparticles and preventing coagulation, when used before the swellingstep.

A higher polymer solid level will increase the need for surfactant. Aneffective amount of the surfactant can be 0.1-3% by weight of themonomer charged, in some cases no surfactant is needed. Advantageously,the anionic surfactant is charged in equal portions in equal periods oftime.

The obtained final product contains very low concentrations of residualmonomers, which is due to the reaction conditions of this process.

The hiding power of hollow particle polymer latexes is determined by thefollowing method. The hiding power can be expressed by the amount ofgrams of the pigment for a surface of 1 m² using a photometric method. 4to 5 glass plates are coated with a latex composition (hollowparticles+film forming latex). The reflection coefficient of the coatedglass plate at λ=582 nm is determined using a photometer. The glassplate is placed on a white surface with a reflection coefficient of 86%and then a black surface with a reflection coefficient of less than 1%.Then, the contrast coefficient R is calculated as the ratio ofreflection coefficients. The hiding power is calculated from therelationship of R to the weight content of the pigment for a surface of1 m². The weight content of the pigment (PWC) value relevant to R=0.98is taken for hiding power.

The diameter of the hollow particle is determined experimentally as wellas the wall thickness.

The hollow polymer particle latexes of the present invention may be usedin coating formulations or coating systems to replace all or part of theopacifying pigments.

The hollow polymer particle latex may be formulated to a composition forthe preparation of white and opaque coatings, for example, in thefollowing manner:

Hollow particle latex with solids 28-32% 4.5-5.0 g Film forming latex(latex of copolymer of 7.5 g styrene-acrylic acid ester-acrylic acid)with minimum film formation temperature from 10-15° C., with particlesize 200-300 μm Water 2.0 to 4 g Ammonia solution (7.2%) 0.2-0.3 g

The following examples are illustrative embodiments of the presentinvention and are not meant to narrow the scope of the process of theinvention.

EXAMPLE 1

The process is carried out in a 200 cm³ reactor equipped with anchortype stirrer, reflux condenser, nozzle for the inert gas input and adevice for monomer feeding at 80° C. under constant values of stirringand nitrogen flow.

Stage (a): 0.0018 g of sodium dodecylbenzene sulfonate (SDBS) dissolvedin 49.9 g of water is charged into a reactor and a nozzle for gas isconnected. SDBS solution is heated to 80° C. for 30 min. Then 0.092 g ofpotassium persulfate (PP) dissolved in 6.3 g of water is charged. Within15 min., a monomer mixture consisting of 0.453 g methyl methacrylate(MMA) and 0.0088 g methacrylic acid (MAA) is charged. The processcontinues after charging for 1 hour and results in the formation oflatex with 100 nm particle size containing 1.18% solids.

Stage (b): 0.0046 g of SDBS dissolved in 2.3 g of water is charged intothe reactor containing the whole amount of the latex prepared at stage(a). After 10 minutes, feeding of the monomer mixture consisting of3.624 g MMA, 1.502 g MAA and 0.031 g ethylene glycol dimethacrylate(EGDM) is started and it is then continued for 3 hours at a constantrate followed by the process for half an hour.

Stage (c): An aqueous solution of SDBS and 0.101 g of PP dissolved in7.43 g water is added to the latex prepared at stage (b). In 10 minutes,feeding of monomer mixture consisting of 12.002 MMA, 2.806 g n-butylacrylate (BA), 0.709 g MAA, 0.026 g EGDM is started and then continuedfor 4.5 hours with constant rate. Then the process is continued for halfan hour.

SDBS solution (0.227 g in 8.00 g of water) is charged by 4 equalportions. The first portion is charged at the beginning of stage (c) andevery subsequent portion is charged after 70 minutes from the beginningof the previous charge.

Stage (d): 11.90 g of 4.0% aqueous ammonia solution is charged into thelatex prepared at the stage (c) for 40 minutes.

Stage (e): 0.270 g of PP dissolved in 16.1 g of water is charged intothe latex neutralized by ammonia. In 10 minutes, the feeding of themonomer mixture containing 11.40 g styrene (ST), 3.84 g MMA, and 0.27 gdivinyl benzene (DVB) is started and completed within 3 hours. Then theprocess is continued for half an hour with subsequent cooling of thereaction mixture to 30-40° C. The latex obtained is collected.

Characteristics of the latex prepared:

Solids 26.2% wt Particle diameter 370 nm Hollow diameter 270 nm Residualmonomers Styrene 0.03% wt n-Butylacrylate 0.04% wt Methylmethacrylate0.007% wt

The characteristics of the coating based on hollow particle latex andfilm forming latex (ST:BA:glycidil methacrylate:MAA=26.7:57,5:10.0:5,8%wt):

Hiding power of the coating: 60.5 g of hollow pigment/m².

EXAMPLE 2

The process is carried out in 350 cm³ reactor equipped with an anchortype stirrer, reflux condenser, nozzle for the inert gas input and thedevice for monomer feeding at 80° C. under constant values of stirringand nitrogen flow.

Stage (a): 0.031 g of SDBS dissolved in 120 g of water is charged intothe reactor and the nozzle for gas is connected. SDBS solution is heatedto 80° C. for 30 minutes. Then, monomer mixture consisting of 14.82 gMMA and 0.28 g MAA is charged. Within 10 min, 0.134 g of PP dissolved in15 g of water is charged. The process continues after charging for 7hours and results in the formation of latex with 90 nm particle sizecontaining 9.70% wt of solids.

Stage (b): 10.11 g of seed latex prepared at stage (a) containing 80.4 gof water is charged into the reactor, stirring is started and the nozzlefor gas is connected. Seed latex is heated to 80° C. for 40 min. Then0.045 g of PP dissolved in 25 g of water is added to the latex, andafter 10 minutes, feeding with monomer mixture containing 6.74 g MMA,2.84 g MAA. and 0.06 g EGDM is started. In 2.5 hours from the beginningof the process 0.045 g of PP dissolved in 2.5 g of water is additionallycharged. Feeding continues for 5 hours followed by the process foranother 1.5 hours.

Stage (c): Aqueous solution of SDBS and 0.22 g of PP dissolved in 6.45 gof water is added to the latex prepared at the stage (b). After 10minutes, feeding of the monomer mixture consisting of 23.88 g MMA, 5.68g BA, 1.42 g MAA, 0.043 g EGDM is started, which then lasts for 4.5hours at a constant rate. Then, the process is continued for anotherhour.

An SDBS solution containing 0.48 g in 8 g of water is charged with 4equal portions. The first portion is charged at the beginning of thestage (c), then every subsequent portion is charged after 70 min fromthe beginning of the previous charge.

Stage (d): 29.4 g of 3.6% aqueous solution of ammonia is charged intothe latex prepared at the stage (c) for 40 minutes.

Stage (e): 0.39 g of PP dissolved in 20.15 g of water is charged intothe latex neutralized by ammonia. After 10 minutes, the feeding of themonomer mixture containing 18.86 g ST, 6.48 g acrylonitrile (AN) and0.48 g DVB is started and completed within 3 hours. Then, the process iscontinued for 1.5 hours with subsequent cooling of the reaction mixtureto 30-40° C. and the latex obtained is collected.

Stage (f): 15.4 g of 3.5% aqueous solution of ammonia is charged intothe latex prepared at the stage (e) for 15 m in at 80° C. Then, 0.28 gof SDBS and 0.36 g of PP dissolved in 28.5 g of water is added. After 10minutes, the monomer mixture containing 3.82 g ST, 6.74 g BA, 0.57 gMMA, and 0.23 g EGDM is added. After the charging is completed, theprocess continues for 40 min. The latex is cooled and collected.

Characteristics of the latex prepared:

Solids 27.62% wt Particle diameter 430 nm. Hollow diameter 280 nm.Residual monomers Styrene 0.006% wt n-Butylacrylate 0.024% wt Hidingpower of the coating: 76.1 g of hollow pigment/m².

EXAMPLE 3

The process is carried out according to that described in Example 2 withthe following differences and additions:

Stage (a) recipe:

Stage (a) recipe: MMA 14.82 g MAA 0.28 g SDBS 0.02 g PP 0.134 g Water135 g Solids 9.76% wt Particle diameter 100 nm

Stage (c) Recipe Latex prepared at stage (b) MMA 18.43 g ST 4.60 g BA5.26 g MAA 1.36 g EGDM 0.05 g SDBS 0.43 g PP 0.19 g Water 15.6 g

Stage (d) Recipe Latex prepared at stage (c) Ammonia 3,5% wt aqueoussolution 21.9 g

Stage (e) Recipe Latex prepared at stage (d) ST 28.17 g MMA 9.41 g DVB0.76 g PP 0.74 g Water 75.0 g

Stage (f) Recipe Latex prepared at stage (e) ST 1.39 g BA 2.45 g MMA0.21 g EGDM 0.05 g Ammonia 3,5% wt aqueous solution 5.6 g SDBS 0.10 g PP0.13 g Water 10.4 g

Characteristics of the latex prepared

Solids 33.9% wt Particle diameter 560 nm Hollow diameter 310 nm Hidingpower of the coating: 71.1 g of hollow pigment/m² Residual monomers:Methyl methacrylate 0.010% wt, n-Butylacrylate 0.012% wt, Styrene0.0333% wt.

EXAMPLE 4

The process is carried out according to that described in Example 2 withthe following differences and additions.

Stage (a) is carried out according to Example 3.

Stage (b): Recipe Seed latex 11.33 g MMA 7.56 g MAA 3.18 g EGDM 0.07 gPP 0.11 g Water 101.5 g

Feeding lasts for 7 hours followed by the process for another 1 hour.

Stage (c): Recipe Latex prepared at stage (b) MMA 29.37 g BA 6.77 g MAA1.74 g EGDM 0,07 g PP 0.26 g SDBS 0.55 g Water 21.46 g

Stage (d): 27.8 g of ammonia 3.8% aqueous solution is charged.

Stage (e): Recipe Latex prepared at stage (d) ST 25.85 g AN 8.50 g DVB1.39 g PP 0.69 g Water 21.86 g

Characteristics of the latex prepared:

Solids 25.5% wt. Particle diameter 500 nm Hollow diameter 300 nmResidual monomers Styrene 0.04% wt Divinylbenzene 0.003% wt

Hiding power: 75.4 g of hollow pigment/m²

EXAMPLE 5

The process is carried out according to that described in Example 4 withthe following differences and additions:

Stage (a): Recipe MMA 14.84 g MAA 0.28 g SDBS 0.016 g PP 0.134 g Water135 g Particle diameter 120 nm

Stage (d): 22.5 g of ammonia 3.8% aqueous solution is charged.

Characteristics of the latex prepared:

Solids 26.8% wt Particle diameter 500 nm Hollow diameter 370 nm Residualmonomers: Styrene 0.04% wt Divinylbenzene 0.003% wt Hiding power: 66.7 gof hollow pigment/m²

EXAMPLE 6

The process is carried out according to that described in Example 4 withthe following differences:

Stage (a) is carried out according to Example 2.

Stage (b): Recipe Seed latex 8.60 g MMA 7.82 g MAA 2.54 g EGDM 0.051 gPP 0.084 g SDBS 0.006 g Water 60.8 g

Stage (c) Recipe Latex prepared at stage (b) MMA 26.72 g BA 6.10 g MAA1.57 g EGDM 0.06 g SDBS 0.50 g PP 0.22 g Water 18.0 g

Stage (d) Recipe Latex prepared at stage (c) 23.7 g of ammonia 3.8%aqueous solution

Stage (e) Recipe Latex prepared at stage (d) ST 32.68 g MMA 10.91 g DVB0.35 g PP 0.86 g Water 76.2 g

Characteristics of the latex prepared:

Solids 33.9% Particle diameter 560 nm Hollow diameter 280 nm Residualmonomers Styrene 0.03% wt n-Buthylacrylate 0.018% wt Methylmethacrylate0.013% wt Hiding power: 76.0 g of hollow pigment/m²

Comparative Example.

Preparation of latex according to U.S. Pat. No. 4,427,836

The preparation of seed latex.

The process is carried out in a 500 cm³ flask equipped with a stirrer,reflux condenser, nozzle for the inert gas input and a device forfeeding monomers. 290 g of distilled water and 0.20 g of SDBS are heatedin the flask to 78° C. under nitrogen atmosphere with stirring. Themonomer emulsion is prepared using 27 g of water, 0.04 g of SDBS, 41.6 gof BA, 37.4 g of MMA and 1.04 g of MAA. 5 g of the emulsion is added tothe flask. Then, 0.3 g of ammonium persulfate is dissolved in 1 ml ofwater. After 15 minutes, the feeding of the remaining monomer emulsionis started with the rate of 1.6 gl/min. 15 min after the monomer feedingis completed, the reaction mixture is cooled. Then 0.14 cm³ of t-butylhydroperoxide (50%) and 0.05 g of sodium formaldehyde sulfoxylatedissolved in 2 cm³ of water are added at 55° C. Then, 1.16 g of 24%aqueous solution of ammonia is added at 25° C.

Stage A.

0.42 g of sodium persulfate (SP) dissolved in 2.5 g of water is added to211.5 g of water heated to 84° C. in the 500 cm³ flask under nitrogenatmosphere, followed by the addition of 6.2 g of seed latex. A monomeremulsion consisting of 23.5 g of water, 0.08 g of SDBS, 49 g of MMA, 21g of MAA and 0.35 g of ethylene glycol diacrylate is added to the flaskat 85° C. over a 3 hour period. After the completion of the monomerfeeding, the dispersion is held at 85° C. for 30 min and cooled to 25°C.

Stage B.

0.21 g of SP followed by 13.5 g of latex obtained at Stage A are addedto 240 g of water heated to 85° C. in a stirred reaction flask. 54 g ofMMA is added over a 1 hour period at 85° C. The temperature ismaintained at 85° C. for another 1.5 hours. Then a monomer mixtureconsisting of 30 g of BA and 24 g of MMA is charged during a 1 hourperiod. The heating is continued for another 2 hours. Then, the latex iscooled to 25° C.

Stage C.

Latex obtained in Stage B is neutralized with 17.4 g of 10% aqueoussolution of ammonia at 25° C. under stirring. The neutralized latex isheated at 95° C. for 1 hour, and then cooled to 25° C.

The latex prepared is film forming at ambient temperatures. Therefore,the hiding power measurements were carried out without the addition offilm forming latex. It appeared to be difficult to measure the hidingpower using our techniques because the contrast ratio (R) was too loweven for thick coatings. For example, R is equal to 0.547 at 350 g/m²surface density of polymer pigment.

What is claimed is:
 1. A process for preparation of hollow polymerparticle latexes by emulsion copolymerization with water solubleinitiator and anionic surfactant, characterized in the following stages:a) preparation by emulsion polymerization of a seed latex of a copolymercontaining methyl methacrylate and methacrylic acid monomer units; b)preparation by emulsion polymerization of a latex of a highlycarboxylated copolymer containing methyl methacrylate monomer units,monoethylenically unsaturated carboxylic acid monomer units and across-linking agent and optionally containing vinyl aromatic monomerunits, in the presence of the seed latex prepared in stage(a); c) in thepresence of the preparation by emulsion polymerization of anintermediate shell comprising a copolymer of methyl methacrylate, anacrylic acid ester, a monoethylenically unsaturated carboxylic acid anda cross-linking agent, and optionally containing vinyl aromatic monomerunits, wherein said intermediate shell copolymer is prepared in thepresence of the highly carboxylated latex synthesized at stage (b), andhas a glass transition temperature below 80° C.; d) swelling theparticles prepared at stage (c) by the addition of a volatile basiccompound; e) preparation by emulsion polymerization of a hard shell inthe presence of the swollen expanded particles prepared in stage (d),said hard shell comprising a copolymer of a vinyl aromatic compound,acrylonitrile and/or methyl methacrylate, and a cross-linking agent,said hard shell having a glass transition temperature above 80° C.; andf) optional preparation of an external shell.
 2. Process according toclaim 1, characterized in that the monoethylenically unsaturatedcarboxylic acid in both stages (b) and (c) is methacrylic acid. 3.Process according to claim 1, characterized in that the seed latexprepared at stage (a) has particle size of from 70 to 180 nm.
 4. Processaccording to claim 1, characterized in that stage (b) comprises chargingthe water soluble initiator as two equal portions in the beginning ofthe reaction and in the middle of the process.
 5. Process according toclaim 1, characterized in that stage (c) comprises charging the anionicsurfactant as equal portions in equal periods of time.
 6. Processaccording to claim 1, characterized in that the ratio of weightconcentrations of the sum of methyl methacrylate and vinyl aromaticcompound to acrylic acid ester at stage (c) is within the range of3.3-4.3.
 7. Process according to claim 1, characterized in that theconcentration of volatile basic compound at stage (d) is calculated inaccordance with the following equation: [B]=a/D−b where [B] is thevolatile base concentration calculated as milliequivalents per 1 g ofmethacrylic acid charged at stages (a), (b) and (c), D is number averagediameter of the seed particles (in nanometers) prepared at stage (a) anda and b are constants, a=1670, b=4.1.
 8. Process according to claim 1,characterized in that additionally after stage (e), stage (f) isperformed, comprising preparation by emulsion polymerization of anexternal shell consisting of a copolymer of a vinyl aromatic compound,an acrylic acid ester, methacrylic acid and a cross-linking agent in thepresence of the latex particles prepared in stage (e), said externalshell having a glass transition temperature below 15° C.
 9. Processaccording to claim 8, characterized in that the volatile baseconcentration at stage (f) is equal to 20-80 meq per 1 g of methacrylicacid charged at stage (f).
 10. Process according to claim 8,characterized in that the weight ratio of the copolymer prepared atstage (f) to the copolymer prepared at stage (e) is 0.1-1.0.
 11. Processaccording to claim 1, characterized in that the temperature for thepreparations is 60-95° C.
 12. Process according to claim 1,characterized in that the process is performed in one reactor withoutisolation of intermediate products or it is performed stagewise with theisolation of intermediate products, in more than one reactor. 13.Process according to claim 1, characterized in that stage a) comprisesthe preparation of a seed latex of a copolymer containing methylmethacrylate and methacrylic acid mononumeric units; wherein the weightratio of monomers/initiator/water during polymerization is equal to1/(0.1-0.3)/50-150).
 14. A coating formulation, characterized in that itcomprises a latex prepared according to claim 1.